[compiler-rt][UBSan] Sanitization for alignment assumptions.

Summary:
This is the compiler-rt part.
The clang part is D54589.

Reviewers: filcab, vsk, #sanitizers, vitalybuka, rsmith, morehouse

Reviewed By: morehouse

Subscribers: rjmccall, krytarowski, rsmith, kcc, srhines, kubamracek, dberris, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D54590

llvm-svn: 351106